So probably about 5 years (they are being advanced all the time, a lot of R&D)

We can replace them, but it may require some tools and skill. Moreover, new batteries can be hard to find and expensive as they often have a specific packaging.
This is what I find so incredibly annoying. They could have used a 4xAA bay. Reader doesn't really use much juice unlike PDAs that have to power their screens all the time. Then it would have been trivial and cheap to replace batteries at any time.

The main advantage of a LiIon pack is that it is smaller, can be made in a slim packaging that fits in the gizmo and takes minimum space. AAs are bit bigger and require more volume, which in a small slim gizmo such as Reader, they think, is a bad thing. In effect, you'd have a bump at the bottom for the extra space required by AAs. From my perspective it doesn't matter, but ....

So they use LiIon to keep a nice slim case design and ignore battery replacement issue. As many have said, a lot of users will toss it in 5 years to get a newer version.

As it is, I'll wait for a newer version using AAs. Otherwise like someone said, I'll purchase this Reader version from eBay in 5 years when people throw them away for $10 because of dead batteries. Then I'll get one for $10 and bolt on a custom made power supply using AAs.